In many cases a network element of a data transfer network is modular so that the network element comprises control entities and controllable entities. The network element can be for example an Internet Protocol “IP” router, a MultiProtocol Label Switching “MPLS” switch, a packet optical switch, an Ethernet switch, and/or a software-defined networking “SDN” controlled network element. Each of the control entities can be for example a physical control unit “CU” of the network element or a control process to be run in the network element such as a routing protocol process e.g. a protocol independent multicast—sparse mode “PIM-SM” process. Each of the controllable entities can be for example a line interface unit “LIU” e.g. a line card, a software-based forwarder process, or a virtualized network function. The control entities are adapted to configure the controllable entities so that the network element constitutes a part of the data transfer network in a desired way. The control entities can be adapted to run for example control-plane functionalities and the controllable entities can be adapted to run for example data-plane functionalities in accordance with the configuration received from the control entities.
An inherent advantage of having many control entities is that the control entities can be adapted to backup each other. On the other hand, an inherent challenge related to this approach is the need to ensure that configuration data-sets maintained by different ones of the control entities are sufficiently congruent with each other. The configuration data-set may indicate for example the allocation of hardware “HW” resources and/or software “SW” implemented resources of controllable entities for implementing desired forwarding functionalities. Each controllable entity, e.g. a line interface unit “LIU”, controlled by a control entity should be able to continue its operation without too big disturbances after a switchover where the above-mentioned control entity has been replaced by another control entity for controlling the controllable entity under consideration. A typical approach for maintaining sufficient congruency between the above-mentioned configuration data-sets is a synchronization mechanism where backup control entities receive update data from a primary control entity so that the backup control entities are enabled to keep their configuration data-sets sufficiently congruent with that of the primary control entity. For large configuration data-sets, a synchronization mechanism of the kind mentioned above can be error-prone and time consuming, and furthermore changes in the configurations of the controllable entities can be blocked when the synchronization mechanism is in operation. For near real-time applications such blocking is not desirable and should be avoided. For example, when the PIM-SM is used for implementing a video service, a user may feel it unsatisfactory if changing a channel takes a long time because of being temporarily blocked by a synchronization mechanism of the kind mentioned above.